It is not stupid 
By default matomo will NOT record/listen/monitor any request from another domain where the site id for a domain is not defined. Which means matomo will monitor only the domains you have set up
In the admin at /System/General Settings/ you have a " Cross-Origin Resource Sharing (CORS) domains" which allows you to add domains or sub-domains you want matomo to monitor too.
So if you did not put any domain in there (CORS) matomo will NOT monitor domains that are not in its database.